The International Association for the Study of Pain has defined pain in the following way: “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” [1]. The problem, however, is that pain cannot be measured directly in animals, but can only be estimated by examining their responses to nociceptive stimuli. Most models of nociception are based on behavioral responses to pain, ranging from the most elementary motor reflexes to far more integrated behaviors (e.g. escape, avoidance).

Distention of the viscera is a natural occurring noxious stimulus in clinical visceral pain, and has been used as a noxious mechanical stimulus in several animal studies. It produces avoidance behavior, as well as abdominal muscle contractions (visceromotor response), increased mean arterial pressure (MAP) and tachycardia (pseudoaffective response) in the conscious rat. These abdominal cramps can be recorded by electromyography (EMG). Several research groups have measured this parameter in the conscious or lightly anaesthetised rat by inserting electrode wires into the abdominal musculature. These were exteriorized on the back of the animal, from where they could be connected to an ink-writer or computer for EMG analysis.

Our group was interested in developing a new technique to measure EMGs, MAP and body activity simultaneously in the conscious, freely moving rat during duodenal distention. Therefore, a telemetry transmitter, consisting of a bipolar electrode pair and a blood catheter, was chronically implanted into the abdominal cavity of the rat. The tips of the electrodes were sutured into the abdominal muscle for EMG measurements, and the blood catheter was placed into the abdominal aorta to register MAP. A balloon catheter was also chronically implanted into the duodenum and tunnelled subcutaneously to the skull, where it was fixed to a connector. During the experiment, a fluid-filled long-line connected to a syringe was attached to the balloon catheter, so that variable volume distention could be delivered from outside the home cage. Body activity was measured by detecting changes in signal strength from the transmitter as the animal moved about its cage.

The advantage of this telemetry system is that behavioral, autonomic and visceromotor responses can be measured at the same time without additional handling procedures or external connection lines. We will discuss how EMGs and cardiovascular parameters can provide us with valuable information about the degree of visceral pain perception in the conscious animal.

References

1. Mertz, H. (1979). Pain terms: a list with definitions and notes on usage. Pain, 249-252.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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